The present invention relates to an extrusion head for a blow molding machine for producing multi-layered hollow plastic bodies, with one or more extruders, attached to the extrusion head housing, for feeding molten plastic melt into the extrusion head, with a central torpedo, and at least one distributor element in concentric surrounding relationship to the torpedo for distribution of the plastic melt(s) across the circumference and feeding in an annular storage space which terminates in an annular outlet nozzle through which the plastic melt can issue.
Extrusion heads (=storage heads or accumulator heads) for blow molding machines are generally known.
U.S. Pat. No. 5,256,051 (Mauser) shows in FIG. 1 an extrusion head with two concentric spiral-channel distributor sleeves. The outer spiral-channel distributor is additionally covered by a further cylindrical sleeve. All three sleeves form the ejector piston which is configured for displacement in an axial direction and expels the accumulated plastic material from the storage space through the lower ring nozzle. In this extrusion head, both plastic streams are distributed in a disadvantageous way across the circumference via diameters of comparably very different sizes so that a wide construction of the extrusion head is required with the additional cover sleeve.
It is an object of the present invention to provide a novel extrusion head for a blow molding machine for producing two-layered hollow plastic bodies, in particular of large volume, which has rheologically substantially same flow conditions for all plastic particles during supply and circumferential distribution of the different molten plastic melts to the distributor elements, and in the distributor elements themselves, while at the same time realizing an improved overlap and circumferential distribution of the melt streams in the distributor element and a trouble-free homogenization in the storage space at simultaneous increase of the throughput power and production speed of the blow molding machine. Hereby, the structural height and the dimension of the diameters should be kept as small as possible.
According to the invention, this object is attained by providing two distributor elements in confronting relationship in an annular gap inside on the inner surface of an outer cylindrical sleeve and outside on the outer surface of an inner cylindrical sleeve centrally within the ejector piston, with the circumferentially distributed plastic material of both distributor elements exiting at the lower end of the ejector piston. This advantageous construction results in an extrusion head of slightest diameter and short structural height while realizing great throughput power.
According to a configuration of the invention, a comparably thin cylindrical separation sleeve is arranged approximately centrally within the annular gap so that both distributor elements are separated functionally but yet are disposed closely next to one another in a spatially very tight or coaxial manner. Thus, both circumferentially distributed plastic streams exit at the lower end of the annular gap from the ejector piston into the storage space at nearly same diametersxe2x80x94at same rheological flow conditions.
In accordance with a further advantageous configuration, the distributor elements are designed as cylindrical spiral-channel distributors arranged on the outer wall of the inner tubular piece and on the inner wall of the outer tubular piece. Hereby, the spiral-channel distributors have each at least four or more spiral channels spaced evenly across the circumference, whereby the depth of the spiral channels steadily decreases in flow direction with increasing length of the spiral channels, with the spiral channels being fed individually with molten plastic melt from atop of the ejector piston.
The particular configurations of the extrusion head according to the invention with cylindrical circumferential distributors have the following features and advantages:
the distributor elements arranged on the outer wall and inner wall of both tubular pieces may each be configured as ring-channel distributors, hanger-type distributors, heart-shaped distributors, spiral-channel distributors or as suitable combination thereof,
the distributor elements are arranged within the axially movable ejector piston formed of two concentric tubular pieces,
in a configuration as spiral-channel distributor, each individual distributor has at least four or more spiral neighboring channels spaced evenly across the circumference,
the depth of the spiral channels steadily decreases in flow direction with increasing length of the spiral channels,
the individual spiral channels of each of the distributors are supplied or fed from above individually or separately within the ejector piston with partial streams of molten plastic melt from the main plastic melt of the connected extruder,
for all plastic particles in the respective partial plastic streams of both cylindrical circumferential distributors, the flow conditions are rheologically the same to a greatest possible degree, thereby eliminating or evening out the adverse memory effect of the plastic materials,
the extended separation sleeve projecting into the storage space and disposed between the distributor elements prevents both melt streams from adversely influencing one another,
the displacement body at the lower free end of the separation sleeve attains in the lowermost position of the ejector piston a targeted material conduction while completely flushing the edge material when carrying out a material change and/or color change.